Nonwoven webs find application in a number of end uses, including paper towels, disposable diapers, filtration products, disposable wipes, and the like. Nonwoven products of fabrics, comprise loosely assembled webs or masses of fibers bound together with an adhesive binder. It is known to form bonded nonwoven fabrics by impregnating, printing or otherwise depositing an adhesive bonding composition on a base web predominantly comprising relatively long fibers, including those of textile length of from about ½ inch (1.27 cm) to about 2½ inches (6.35 cm), or more. These fibers may be of cellulosic or polymeric materials such as polyesters, polyamides, polyacrylates, and the like. The base web of nonwoven fibers, to which the binder is applied, can be produced by carding, garnetting, airlaying, papermaking procedures, or other known operations. The operation of bonding fibers in place is much less expensive than conventional spinning and weaving. In comparison with woven fabrics, the bonded nonwoven fabrics can be made in a much greater range of thicknesses per unit weight, with more homogeneous structures, no unraveling tendency, and with greater water absorbency, porosity and resiliency, when required.
There are many factors that lead to acceptable nonwoven products. Two major factors are the wet tensile strength and “feel” of the nonwoven product. Personal care products such as tissues, handwipes and sanitary napkins must have sufficient wet tensile strength to remain intact when wet and sufficient softness or feel when in contact with the skin. However, to achieve desirable or sufficient wet tensile strength it has been common practice to elevate the dry tensile strength of the polymer or use higher add-on levels of polymer. Higher dry tensile strengths in a nonwoven product tends to impart stiffness or a hardness to the product and uncomfortable to the touch. Higher add-on levels of polymer are undesirable from a user standpoint in terms of feel and from a cost standpoint.
There has been an industry correlation of the wet tensile strength of a nonwoven product to its dry tensile strength with the dry tensile strength of a nonwoven product generally being secondary to its wet tensile strength. Because of these contrasting properties, i.e., wet tensile strength vs. dry tensile strength and feel, a product that has a high wet/dry tensile strength ratio is desired. Also, a high ratio of wet tensile strength to dry tensile strength generally permits a lower add-on level of polymer to the nonwoven product, thereby improving the feel of the fabric and reducing manufacturing costs
Representative of various binder compositions used in the art include:
U.S. Pat. No. 3,081,197 (1963) discloses a nonwoven binder comprising polymers of vinyl acetate, another polymerizable compound as an internal plasticizer, and a post-curable comonomer such as N-methylol acrylamide.
WO 02/079270 A2 (2002) discloses emulsion polymerized ethylene-vinyl acetate polymers having greater than 55 percent by weight ethylene and a hydrophilic colloid stabilizer. Examples of hydrophilic polymer colloids are based upon at least two copolymerized monomers with one having acidic functionality, e.g., a carboxylic acid or dicarboxylic acid, a sulfonic acid, or phosphonic group. They are characterized as having excellent peel and shear properties with excellent tack. These copolymers are characterized as water-dispersible in an aqueous solution but non dispersible in a solution containing 0.5 with percent of an inorganic salt. Functionalized copolymers formed at 1100 psi ethylene pressure also are disclosed which are suited for nonwoven applications. N-methylol acrylamide, acrylamide, hydroxyethyl acrylate and ammonium acrylamidylpropyl sulfonate are disclosed as the monomers providing functional groups.
U.S. Pat. No. 3,137,589 (1964) discloses binders comprising a copolymer of an alpha, beta-unsaturated carboxylic acid amide substituted on the nitrogen by at least one methylol group and another unsaturated polymerizable compound.
U.S. Pat. No. 3,380,851 (1968) discloses a binder comprising an interpolymer of vinyl acetate-ethylene-N-methylol acrylamide. The ethylene content is from 5 to 40% by weight.
U.S. Pat. No. 4,449,978 (1984) discloses a process for forming vinyl acetate-ethylene nonwoven binders having reduced formaldehyde emitting content. The crosslinking agent is a mixture of N-methylolacrylamide and acrylamide.
U.S. Pat. No. 5,540,987 (1996) discloses the formation of formaldehyde free and formaldehyde reduced vinyl acetate/ethylene binders for nonwoven products. These binders are formed by emulsion polymerization using an initiator system based upon an organic peroxide and ascorbic acid. The crosslinking agent can be N-methylolacrylamide for nonwovens of reduced formaldehyde and iso-butoxy methyl acrylamide for formaldehyde free nonwovens.